The present invention relates to a technology of estimating power available to be inputted to and outputted from a secondary battery.
Such a technology is disclosed in the following literature: Daijiro Yumoto, Hideo Nakamura, Mayumi Hirota, and Norimasa Ochi, “Method of Estimating Internal State Quantity of Battery using Adaptive Digital Filter Theory”, Automobile Technology Association Spring-Autumn Academic Lecture Handout No. 20035031, May 21, 2003.
In the literature, measured values of current I and terminal voltage V, both of which are observable, are substituted into an adaptive digital filter using a battery model represented by
                    V        =                                                            K                ⁡                                  (                                                                                    T                        2                                            ⁢                      s                                        +                    1                                    )                                                                                                  T                    1                                    ⁢                  s                                +                1                                      ⁢            I                    +                                    1                                                                    T                    1                                    ⁢                  s                                +                1                                      ⁢                          V              0                                                          (        1        )            for estimating values of parameters (internal resistance K and open circuit voltage V0). Then, substituting estimated values {circumflex over (K)} and {circumflex over (V)}0 of the internal resistance K and the open circuit voltage V0, and a lower limit value Vmin of the terminal voltage V into a steady-state battery model given byV=KI+V0  (2)allows a value (estimate value) of current Iout, at which a lower limit voltage value Vmin is reached, to be calculated by
                                          I            ^                    out                =                                                            V                min                            -                                                V                  ^                                0                                                    K              ^                                .                                    (        3        )            This permits a value of available output power Pout to be estimated by
                                                        P              ^                        out                    =                                                    V                min                            ⁢                                                                I                  out                                                                      =                                                                                                      V                      ^                                        0                                    -                                      V                    min                                                                    K                  ^                                            ⁢                              V                min                                                    ,                            (        4        )            using the calculated current value Îout and the lower limit value Vmin.
Likewise, substituting the estimated values {circumflex over (K)}, {circumflex over (V)}0 of the internal resistance K and the open circuit voltage V0 and an upper limit value Vmax of the terminal voltage V into Eq. (2) allows a value (estimate value) of current Iin, at which an upper limit value Vmax is reached, to be calculated by
                                          I            ^                    in                =                                                            V                max                            -                                                V                  ^                                0                                                    K              ^                                .                                    (        5        )            
This permits a value of available input power Pin to be estimated by
                                                        P              ^                        in                    =                                                    V                max                            ⁢                              I                in                                      =                                                                                V                    max                                    -                                                            V                      ^                                        0                                                                    K                  ^                                            ⁢                              V                max                                                    ,                            (        6        )            using the calculated current value Îin and the upper limit voltage value Vmax.
Here, the term “open circuit voltage V0” means a terminal voltage during current shutoff; the term “available input power Pin” an instantaneous maximum input power in a range wherein a value of the terminal voltage V does not exceed the upper limit voltage value Vmax that is predetermined for battery protection; and the term “available output power Pout” an instantaneous maximum output power in a range wherein a value of the terminal voltage V does not exceed the lower limit voltage value Vmin that is predetermined for battery protection.
In general, since a value of the terminal voltage V of a secondary battery momentarily varies during a transitional period (that is, charging and discharging period) and a period until the terminal voltage V is stabilized after the charging and discharging, mere attempt to measure a value of the terminal voltage V results in an inability of accurately detecting the open circuit voltage V0 as needed.
Also, throughout the following description, a symbol “^” denotes an estimate value.